Photographic diffusion transfer processes are well known in the prior art, in particular ones that use, to form the color image, initially immobile dye providing substances capable of providing or releasing diffusible dyes or dye precursors during photographic development. Such diffusible dyes and/or dye precursors, after they have been formed, can migrate to a receiving layer where the final color image is formed.
Briefly, the formation of a color photographic image by means of such initially non-diffusing dye providing substances associated with a light-sensitive silver halide emulsion occurs as follows:
1. In a first step, an imagewise exposed and developable silver halide emulsion is developed by means of an appropriate chemical developing agent thereby obtaining formation of oxidized developing agent and metallic silver in proportion to development.
2. In a second step, the oxidized developing agent interacts with the initially immobile dye providing substance, for example, by a coupling reaction or a cross-oxidation reaction, thereby obtaining formation or release of a diffusible dye or diffusible dye precursor.
It may be observed from the above-described reaction mechanism that if a negative silver halide emulsion is used as the developable silver halide emulsion, one obtains a negative dye image with such initially immobile dye providing compounds. However, in most diffusion transfer processes, one primarily is interested in obtaining a positive dye image. For this purpose, several reversal techniques may be used.
One such reversal technique employs, not a negative emulsion, but a direct-positive emulsion, i.e., an emulsion which is developable in the unexposed areas or, more precisely, an emulsion which yields a silver image in inverse ratio to the exposure received. In this regard, U.S. Patent application Ser. No. 351,673, published Jan. 28, 1975 as Trial Voluntary Protest No. B351,673, describes various dye release compounds which contain alkali-cleavable sulfonamido groups upon oxidation thereof and which can be used in conjunction with a positive working emulsion to obtain a positive dye image.
Another such reversal technique described, for example in British Pat. No. 904,364 (page 19, lines 1-41), employs a photographic element having a negative emulsion layer and an adjacent physical development layer containing physical development nuclei and an associated dye providing substance. If, after exposure of the negative emulsion layer of such a photographic element, the element is developed with an appropriate chemical developer in the presence of a silver halide solvent, one obtains (by the now well-known silver-salt-diffusion transfer process employing the diffusion of solubilized, unexposed silver halide) the formation of a positive silver image and a positive dye image in the physical development nuclei layer. The resultant image dye is itself diffusible and therefore, for example, can migrate to a dye image receiving layer.
The aforementioned reversal technique using physical development nuclei is interesting because it makes possible the practice of a diffusion transfer process which employs a negative silver halide emulsion. Negative emulsions are less difficult to manufacture than direct-positive emulsions which are very delicate. In particular, it is difficult to manufacture direct-positive emulsions which give reproducible results, and it is especially difficult to obtain direct positive emulsions having sensitivities comparable to those of negative emulsions.
However, the above-described diffusion transfer process using physical development nuclei nevertheless has some drawbacks when applied to color diffusion transfer processes. For example, in such a process it is difficult to correctly control and correctly carry out a sequence comprising successively the step of negative emulsion development and the steps of silver salt diffusion, physical development and dye formation. In particular, it has been noted that the sensitivity of such a photosensitive element using physical development nuclei may be limited because the dissolution of the silver halide is too rapid. In other words, the silver halide solvent begins to dissolve the silver salts in the unexposed areas of the negative emulsion before the negative image has been developed. Of course, it is possible to break down the processing into two, separate operations, one operation consisting of developing the negative image in a non-solvent medium, and the other operation consisting of the physical development of the remaining unexposed silver halide in a solvent medium. However, such processing is complicated and its duration is lengthened.
Consequently, to improve the sensitivity of a diffusion transfer color photographic element employing physical development nuclei without complicating the processing thereof, it appears desirable to slightly retard or defer the dissolution of the silver salts in the negative emulsion layer. Therefore, the present invention has for one of its objects the elimination or correction of the aforementioned drawbacks as well as providing a generally improved diffusion transfer process of the type which employs a photographic element containing at least one negative silver halide emulsion layer associated with physical development nuclei and with a dye providing substance, such as a sulfonamido compound.